An example of a system that restrains an occupant of an automobile at the time of collision may be an occupant restraint device that prevents a submarine phenomenon from occurring in which even though the occupant fastens a seat belt, the occupant may slip through the lower side of a lap belt at the time of front collision. Japanese Unexamined Patent Application Publication No. 10-217818 discloses an occupant restraint device having an inflatable bag disposed between a seat cushion and a seat pan. The bag is inflated at the time of vehicle collision, so that a front portion of the seat cushion is pushed up.
FIGS. 5a and 5b are cross-sectional views taken along a front-back direction of a seat, each showing the occupant restraint device disclosed in the above-mentioned publication. FIG. 5a illustrates a bag in a not-inflated state, and FIG. 5b illustrates the bag in an inflated state. In the front portion of the seat, an airbag 34 is disposed between a cushion frame (seat pan) 30 and a seat pad (seat cushion) 32. The airbag 34 extends in a left-right width direction of the seat, and is inflatable by way of an inflator (gas generator) 36. The airbag 34 is folded, and disposed above the inflator 36. An upper surface of the seat pad 32 is covered with a trim cover 38, and an occupant sits thereon.
In the occupant restraint device disclosed in Japanese Unexamined Patent Application Publication No. 10-217818, a recessed portion 40 is formed at the seat pan 30, and a deep portion 42 is formed at the bottom of the recessed portion 40, the bottom of the deep portion 42 being one step lower than that of the recessed portion 40. The inflator 36 is disposed in the deep portion 42, and the airbag 34 is disposed in the recessed portion 40.
When the inflator 36 is actuated at the time of vehicle collision, the airbag 34 is inflated, and the forward movement of the body of the occupant is prevented (or restricted) because the front portion of the seat pad 32 is pushed up, or because the density of the front portion of the seat pad 32 is increased by being pushed from below.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 10-217818        
In Japanese Unexamined Patent Application Publication No. 10-217818, since the airbag 34 is folded and disposed, the folding work may be troublesome. In addition, when the airbag 34 is inflated, the airbag 34 may receive a relatively large frictional resistance from the seat cushion 32. Accordingly, the output of the inflator 36 is required to be large by the amount of frictional resistance.
Also, in Japanese Unexamined Patent Application Publication No. 10-217818, since the airbag 34 is folded, the height of the airbag 34 becomes large, and thus, the depth of the recessed portion 40 is required to be large by the amount of increase in height of the airbag 34.